Easy way to add multiple existing .csproj to a Visual Studio Solution?

A PowerShell implementation that recursively scans the script directory for .csproj files and adds them to a (generated) All.sln:

$scriptDirectory = (Get-Item $MyInvocation.MyCommand.Path).Directory.FullName
$dteObj = [System.Activator]::CreateInstance([System.Type]::GetTypeFromProgId("VisualStudio.DTE.12.0"))

$slnDir = ".\"
$slnName = "All"

$dteObj.Solution.Create($scriptDirectory, $slnName)
(ls . -Recurse *.csproj) | % { $dteObj.Solution.AddFromFile($_.FullName, $false) }

$dteObj.Solution.SaveAs( (Join-Path $scriptDirectory 'All.sln') ) 

$dteObj.Quit()

A C# implementation that produces an executable, which creates a solution containing all unique *.csproj files from the directory and subdirectories it is executed in.

class Program
{
  static void Main(string[] args)
  {
    using (var writer = new StreamWriter("All.sln", false, Encoding.UTF8))
    {
      writer.WriteLine("Microsoft Visual Studio Solution File, Format Version 11.00");
      writer.WriteLine("# Visual Studio 2010");

      var seenElements = new HashSet<string>();
      foreach (var file in (new DirectoryInfo(System.IO.Directory.GetCurrentDirectory())).GetFiles("*.csproj", SearchOption.AllDirectories))
      {
        string fileName = Path.GetFileNameWithoutExtension(file.Name);
        
        if (seenElements.Add(fileName))
        {
          var guid = ReadGuid(file.FullName);
          writer.WriteLine(string.Format(@"Project(""0"") = ""{0}"", ""{1}"",""{2}""", fileName, file.FullName, guid));
          writer.WriteLine("EndProject");
        }
      }
    }
  }

  static Guid ReadGuid(string fileName)
  {
    using (var file = File.OpenRead(fileName))
    {
      var elements = XElement.Load(XmlReader.Create(file));
      return Guid.Parse(elements.Descendants().First(element => element.Name.LocalName == "ProjectGuid").Value);
    }
  }
}